This invention relates to a thermal transfer material which is used in thermal printing equipments such as thermal printers, etc.
More particularly, it relates to a thermal transfer material which permits formation of an image on an image-receiving sheet in a thermal printing equipment in which thermal printing corresponding to image information is conducted by means of a thermal head or the like.
Recently, thermal transfer materials comprising a substrate and a hot-melt ink (hereinafter referred to as "thermal transfer ink") coated thereon have rapidly come to be used in thermal printing equipments such as thermal printers, thermal facsimiles, etc., so that clear images can be formed on ordinary paper.
As the substrate, there is used a plastic film such as a polyethylene terephthalate (PET) film of about 4 to about 6.mu. in thickness, whose surface to be brought into contact with a thermal head has a heat resistant layer for preventing sticking.
As to the heat resistant layer, there are various prior art references.
For example, in Japanese Patent Application Kokai (Laid-Open) No. 56-155794, it is proposed that a sticking-preventing layer (a kind of heat resistant layer) comprising an inorganic pigment having a high lubricity and a resin material which is thermosetting or has a high plasticity or a high softening point should be formed on one side of a plastic film. As the inorganic pigment, there are exemplified, for example, particles having a particle size of about 0.01 to about 5 .mu.m of talc, mica powder, fine silica powder, molybdenum disulfide, etc. As the resin material, there are exemplified silicone resins, epoxy resins, melamine resins, phenolic resins, fluorine-contained resins, polyimide resins, nitrocellulose resins, etc. The ratio of the resin material to the inorganic pigment is about 20: about 80 to about 98: about 2 by weight. The thermal transfer material proposed permits prevention of sticking phenomenon but has the following defects. The surface roughness of the heat resistant layer is too high, so that its heat conduction becomes insufficient, resulting in a very serious adverse effect on the quality of printing. That is to say, there are non-transferred portions in one image plane, or when the same pattern is printed in many copies, the quality of printing is very uneven. Therefore, said thermal transfer material is insufficient to the attainment of a high quality of printing. Particularly in the case of color thermal transfer materials, only one of three colors, yellow, magenta and cyan changes in hue owing to unsatisfactory transfer in some cases. When the particle size of the inorganic particles exceeds 0.35 .mu.m, the image quality is deteriorated, while it is less than 0.15 .mu.m, sticking tends to occur.
On the other hand, U.S.P. 4,675,233 discloses a transfer material for printer using as a base a biaxially oriented polyester film having on at least one side thereof a rough surface having an average centerline roughness of 0.02 to 1 .mu.m and a maximum height of 0.2 to 10 .mu.m. However, this transfer material is poor in stability of image quality and can cause sticking. There has been reported no technical idea that in order to remove these defects, a heat resistant layer is formed on one side of a base film so as to have projections of 0.35 to 1.00 .mu.m high in number of 50 or more per mm.sup.2 on the surface thereof.